Thousands turn out to protest the disputed electoral victory of the premier's party.

MOSCOW — Cheerful and smiling, in groups of friends, lovers and associates, carrying white balloons, flowers and ribbons, they came together in the massive square on an island embraced by the Moscow River.

For most, it was the first time: successful young bank managers and businessmen, computer programmers and engineers, lawyers and real estate agents.

The tens of thousands of protesters who joined forces Saturday in the largest demonstration here since the collapse of the Soviet Union were united in one chant: "Russia without Putin!"

Days earlier, parliamentary elections that saw Prime Minister Vladimir Putin's United Russia party garner nearly 50% of the vote were met with widespread allegations of vote-rigging. As the Obama administration joined the chorus of those questioning the poll's credibility and the Kremlin deemed the elections fair, people who had never before been active in pro-democracy protests were galvanized into action.

Many of them following the call of Facebook and other social networks, the mostly young protesters brought a touch of the "Arab Spring" to this Russian winter day as wet snow turned into a gray drizzle.

"I realized that I can't keep silent anymore when out of about 70 of my friends and relatives, only two voted for United Russia, one of those two being a municipal employee and the other a daughter of [President Dmitry] Medvedev's driver," Leila Arifulina, a 35-year-old fitness instructor, said as she protested in Bolotnaya Square. "And the next morning they tell me the Kremlin party won!

"I am tough and can put up with lots of things in this life, but they shouldn't really have treated me as an idiot."

On Thursday, Putin appeared to wash his hands of the compromised ruling party as he launched his campaign to return to the presidency in March announcing that he would build his campaign based on the United People's Front, a recently created umbrella group.

But the mass protests Saturday demonstrated that Putin is a focus of many Russians' anger, and he may face serious challenges in his campaign if the middle class increasingly turns away from him.

Authorities put the number of protesters Saturday at 20,000 to 25,000, but organizers claimed that at least 60,000 people had filled the square and the surrounding area. Thousands of others demonstrated in St. Petersburg and about 90 other cities and towns Saturday. Earlier in the week, protesters took to the streets and hundreds were arrested, many of them sentenced to up to 15 days' imprisonment.

The Kremlin deployed its elite Dzerzhinsky Interior Ministry division in Moscow, and on Saturday morning the troops were around Bolotnaya Square, along with dozens of military trucks loaded with metal shields and other crowd-control gear. Prison vans were parked in preparation near the square.

But the crowd was not aggressive. There were no clenched fists, unlike previous gatherings of spoiling-for-a-fight nationalists, left-wing revolutionaries or rowdy soccer fans.

The participants in this rally were polite to strangers, though one could see all kinds of flags around, from the communist red to the nationalist black, white and gold.

Some posters and signs were new, even showing a sense of humor. One sign read: "Shame on shameful shame!" A man passed by holding a poster that said: "I didn't vote for these scoundrels! I voted for the other scoundrels!"

A man next to the speakers platform raised high a poster bearing a picture of the Lenin Mausoleum but with the name of Putin instead of Lenin, with the words below: "We believe, wait and hope!"

Nikolai Popov, a 30-year-old designer, said it was the first time he had joined an opposition rally, expressing anger that his vote had been stolen with such impudence.

"I doubt very much that the authorities will hear me, but it doesn't mean that we should shut up and forget about it," he said. "What I want least of all is a revolution, but it is up to the Kremlin now to find a good way to avoid that," he added, pointing his finger toward the Kremlin half a mile away across the river.

Even some speakers Saturday were different from the regular protest leaders. Boris Akunin, an author of historical mystery novels, mounted the platform and acknowledged to an understanding cheer from the crowd that it was his first time too. He demanded a revote with an online live broadcast from every polling station.

"I haven't seen such a Moscow for 20 years, and I thought I will never see it like that again," he said. "Today is a very important day in my life and I believe in your lives too!"

A young man asked an elderly protester whether the rally was bigger than in the last days of the Soviet Union 20 years ago. The older man asked: "Don't you remember?" "Hey, I was only 2 at the time," the young man said, laughing, and they embraced and patted each other on the back, two generations coming together.